Lamp current regulation at normal operation and lamp voltage regulation at open lamp condition is a function that is implemented by a CCFL inverter. Furthermore, the CCFL inverter should implement short circuit protection and open lamp voltage protection.
Typically, the winding current (or lamp current) is sensed for open lamp protection and lamp current feedback. The lamp voltage may also be sensed for short circuit protection and lamp voltage regulation during an open lamp condition. Generally, the feedback circuit and the protection circuit cannot be integrated together due to their differing requirements. For example, the lamp voltage feedback needs the maximum value of the sensed lamp voltage, and the short circuit protection circuit needs the minimum value of the sensed lamp voltage for proper detection. Therefore, four separate sets of circuits for the feedback and protection circuit are required. This requires significant external circuitry and makes for a complex circuit structure. Furthermore, the circuits are even more complex for out-of-phase applications, which are popular in current CCFL inverter systems.
A CCFL exhibits large impedance if current is not applied and smaller impedance once the lamp is ignited. Therefore, during startup period or open lamp condition, the lamp voltage should be regulated to a high value to ignite the lamp. Due to inherent characteristics of the series-parallel resonance of the circuit, the switching frequency at that time should be set to a higher value than during normal operation. In a conventional CCFL inverter, the frequency hop method is usually used. Some controllers provide separate pins to set the normal operation frequency and open lamp frequency respectively. Other controllers simply set the open lamp frequency to a certain ratio of the normal operation frequency internally. Some controllers use external circuitry to implement this function. These methods are usually complex or require an extra pin for the controller. Though the internal frequency hop method is simple, it is not flexible for varying loads. At certain load conditions, it may cause instability of the CCFL inverter.